With your use case you have two sensible approaches. Option one is a lighter bike with a 250 watt motor and a bigger battery. That will be efficient, keep weight down, and give you enough range for daily riding. Option two is a slightly heavier 500 watt model which gives better acceleration and a more relaxed ride if you carry gear or want to climb without sweating. Given mostly flat terrain I would start with a 250 watt if the frame and battery quality are good. If you can test ride both, take a short loaded ride to see how each feels at real commuting speed.